1. Field of the Invention
This invention relates to a composite filter, an observation system for dark field utilizing said composite filter, and a filamentary display device provided with said composite filter.
2. Description of Prior Arts
A technology of detecting an image in a dark field such as a dark night with no moonlight has been recently developed. This technology utilizes starlight as the light source and its detection system contains a multi-channeled plate and a photoelectric conversion element which matches with the starlight source.
It is known that the earth goes through space in which approx. a hundred thousand stars are dispersed, for one day. Although most of the stars emit light, the ultraviolet (UV) rays emitted by new-born stars (age: 100,000-1,000,000 years old) are scattered or absorbed in atmospheric gas, etc. so that the UV rays hardly reach the surface of the earth. On the contrary, the near infrared (IR) rays emitted by stars of approximately 5.times.10.sup.6 years old reach the surface of the earth. The spectrum of light emitted by the stars and received by the earth is shown in FIG. 1 as the curve (i). Accordingly, the photoelectric conversion element employed in the system has, for instance, such photoelectric conversion characteristics as shown in FIG. 1 as the curve (ii).
The observation system utilizing the starlight has been developed and now employed generally in the military and police works because such observation system needs no employment of artificial light.
Since the observation system now utilizes a multi-channeled secondary electron multiplier, the gain is at very high level. The detection device employed for the observation system contains a number of artificial light sources such as a lighting device, various display devices, etc., most of which emit light having a radiation spectrum overlapping in part with the radiation spectrum of starlight. Generally, the luminance of light emitted by these artificial light sources is several millions times to several ten millions times as high as that of the starlight. Therefore, if any light of the artificial light souce is incidentally received in the visual field directed to eyes of the observer, such light not only causes noise but also the highly amplified artificial light causes burning and further destroys the retina of the observer's eyes.